1. Field of the Invention
This invention relates to industrial process instruments for producing an electrical measurement signal corresponding to a pressure or force. More particularly, this invention relates to such instruments of the vibrating-wire type.
2. Description of the Prior Art
It has been well known for many years that a taut wire will have a vibrational resonant frequency related to the tension in the wire. It also has been long recognized that this characteristic can serve as the basis for a pressure or force-measuring instrument, as by tensioning a vibratable wire in accordance with a force to be measured, causing the wire to vibrate at its resonant frequency, and producing a measurement signal corresponding to that frequency. Theoretical considerations applicable to such devices indicate that they should be capable of extremely accurate measurements, and thus substantial effort has been devoted to the development of such apparatus. Out of this effort has come a considerable number of proposals for various kinds of instruments, and some designs have been offered commercially. Moreover, there is substantial patent art relating to such instruments; a selection of relevant disclosures (not herein asserted to be a complete presentation of all such art) may be found in U.S. Pat. Nos. 2,445,021, 3,046,789, 3,071,725 and 3,543,585.
Notwithstanding the extensive work that has been done in this field, there has as yet been no instrument made available which is truly suitable for application to industrial processes. In part, this is because the instruments offered to date have not met important performance requirements, such as reliably achieving high accuracy of measurement. In addition, prior instrument designs of this character were not suited to serve as field-mounted units capable of interconnection into an overall instrumentation system, e.g. of the type having an instrument station where various measurement and/or control functions are coordinated.